1. Field of the Invention
The invention relates to a polyvinyl chloride resin pipe as a material for the inner lining of existing pipes, and more particularly, to a polyvinyl chloride resin pipe serving as an inner lining layer of existing (metal) pipes, wherein the resin pipe is to be inserted in the existing pipes after they have been installed in underground space or in the building body, and wherein the existing pipes serving as a conduit for power source cables, communication wirings or the like are repaired or reinforced by means of such a lining.
2. Description of Prior Art Those existing pipes are apt to sustain unexpected cracks or injuries due to corrosion which will occur in course of time, or due to external pressure imparted to the pipes from surrounding objects Such injured underground pipes will cause a problem such as short circuit, leakage of electricity and breakage of the power source cables or communication wirings, if underground water penetrates into the injured pipes. It is very difficult to replace such existing or established pipes with new pipes since they are located underground below the building or inside the building body.
Therefore, public attention has been attracted to the inner lining method for repairing or reinforcing the existing pipes, which method was proposed recently and in which method a synthetic resin pipe is inserted in the already installed pipes so as to form an inner lining layer therein. According to this inner lining method as described in Japanese Patent Publication Kokai No. 1-295828, a thermoplastic resin pipe whose outer diameter is smaller than the inner diameter of existing pipe is heated to soften and inserted into the existing pipe. Then, compressed hot steam is introduced into the resin pipe in order to charge an internal pressure. The steam expands the resin pipe in its radial direction, so that the resin pipe is secured close to the inner surface of existing pipe. Finally, the resin pipe is cooled down to solidify, by means of an appropriate cooling medium.
A resin pipe formed of rigid polyvinyl chloride resin which has the average degree of polymerization (DP) of about 1,100 to 1,300 has been considered and tried for use as the resin pipe for the inner lining. This type of resin pipe has been widely used to construct the underground pipe line, from the viewpoint of material cost, durability and thermal stability of dimension.
However, it has been recognized that the pipe of rigid polyvinyl chloride resin brings about the following problems when it is heated and inflated by means of heating medium.
The outer diameter of the resin pipe used as the inner lining may usually be about 50 to 95% of the inner diameter of the existing pipe. It is preferable, however, that the outer diameter of resin pipe as the inner lining is designed so small as to be easily inserted into the existing pipe. In this case, the resin pipe must be of a high elongation ensuring diametrical expansion when heated. If steam is introduced into the polyvinyl chloride resin pipe in order to heat and soften the resin, then some inner portions of pipe will be covered inevitably with condensed water and be hindered from being sufficiently heated. On the other hand, some other portions adhering close to the inner surface of existing pipe will be cooled down by the existing pipe itself which absorbs heat. As a result, temperature distribution ranging from 75.degree. to 95.degree. C. will be brought about in the resin pipe. Though any temperatures within this range are not lower than the softening temperature of polyvinyl chloride, such a variance of the temperature will produce unequal wall thickness of the expanded resin pipe, because the peripheral portions of the pipe wall are expanded outwardly in an ununiformed manner. In addition, local excessive expansion will also produce a crack called "burst" of the pipe wall.
Moreover, as another problem, the resin pipe for the inner lining undergoes thermal shrinkage in both the radial and longitudinal directions during the cooling process which is carried out after the heat expansion process of the lining operation. Thus, the radial shrinkage must be prevented by means of the pressure imparted to the inner surface of the resin pipe, whereas the longitudinal shrinkage must be prevented by fixing both ends of the resin pipe to the ends of existing pipe. However such a forcible restriction of shrinkage will inevitably cause the internal strain to remain within the wall of resin pipe, thereby significantly impairing its anti-earthquake and anti-shock strength.